Is there such a thing as "Vulnerable 5 all" ? The invoker power Rain of Blood gives "vulnerable 5 to all damage" which syntactically isn't quite the same thing. To me, it means, if you take damage (regardless of the type) you take an extra 5 damage.
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Pat LudwigSep 8 '10 at 3:43

Whenever a creature suffers a discrete incident of damage, evaluate that damage in isolation against all the creature's vulnerabilities, resistances, triggering effects, etc. etc.

If a single damaging effect has two types, all points of damage are considered simultaneously to be of both types; 5 fire&radiant is not 2.5 fire / 2.5 radiant, and thus is not two separate 'hits.'

Conclusions:

In example 1, the creature suffers a single instance of damage which just happens to deal Thunder and Lightning damage. Evaluate the hit against all the creature's vulnerabilities:

The creature's Vuln 5 Thunder triggers (since the damage is typed Thunder, it meets the criteria for this vulnerability).

The creature's Vuln 2 Lightning triggers (also meets the criteria for this vulnerability).

In example 2a, the creature suffers a single instance of damage which just happens to count as both Fire and Radiant simultaneously. Therefore the "Vulnerable 5 All" gets triggered once (the typing of the damage [Fire&Radiant] meets the criteria for the vulnerability).

In example 2b, the creature suffers a separate instance of Fire damage and Radiant damage - two separate "hits" trigger the Vulnerable rules two separate times, exactly as if two separate instances of example 2a had occurred.

Agreed: good clarification, and I agree with your ruling.
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Viktor HaagSep 3 '10 at 13:32

Good clarification, but I'm not sure of the ruling. How is Example 2a significantly different from Example 1, enough that the rulings are different? I translate "Vulnerable 5 all" as - in relation to the damage types specified in 2a - meaning "Vulnerable 5 Radiant, and Vulnerable 5 Fire". Is this not the correct interpretation?
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IsziSep 3 '10 at 16:32

Hi Iszi, I added my assumptions and the conclusions I draw from them. HTH.
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SkeolanSep 8 '10 at 3:39

I don't think there's a clear ruling on this anywhere, although I'd be happy to be proven wrong. For reference, the Player's Handbook errata includes this note:

"Vulnerability to a specific damage type applies even when that damage type is combined with another. For example, if you have vulnerable 5 fire, you take 5 extra damage when you take ongoing fire and radiant damage."

The Rules Compendium preview gave us this text: "Vulnerabilities to the same damage type are not cumulative." Note that it's calling out "same damage type" explicitly, rather than generally stating that vulnerabilities aren't cumulative.

From that, I would conclude that when an attack does multiple damage types and the target is vulnerable to both, it generally stacks. I.e., in example 1, the creature would take 7 extra damage. However, I wouldn't object to a ruling the other way. See also this thread.

Vulnerable X all is probably a special case; it means that when you take any damage, you take X more points. You don't need to split that into damage types when multiple damage types apply, because it's all-encompassing. This is more subject to argument than the above case, however, and I can't find any cites at all for it.

I'm looking for the rules cites. Well, the main part on the rules on Resistances and Vulnerabilites in the PHB is on page 276. But it doesn't explain this very well.

Vulnerability against the same damage type does not stack (So if you have Vuln 5 to Psychic and you get hit with a Vuln 10 Psychic type effect, you just change to 10, not 15). But the vulnerabilities are applied separately when they are different damage tpes: If it's a Thunder/Lightning attack and you are vulnerable to both thunder and lightning, you take the extra damage from both.

Wow, this specific situation really isn't spelled out in the rules. I know what I'd rule- it seems obvious enough to me that you'd apply both vulnerabilities. But they don't seem to address it either way, except on the reverse principle that if you were resistant to only one facet of a combined damage, that you would still take damage you weren't resistant to. That's where my logic comes from.
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Peter SecklerSep 2 '10 at 17:26

Agreed: I'd use the logic born from 3ed and forward about bonuses here. Bonuses do stack in general, but bonuses of the same type do not. Accordingly, Vulnerabilities do stack, but not if they're of the same type, then you just apply the single biggest one. Accordingly, for example 1 since Vulnerable Thunder and Vulnerable Lightning are separate, they both kick in, even though the damage is from a single source. For example 2, the Vulnerability itself only applies once; it's just triggered by All attack types (and thus wouldn't stack with any other vulnerability).
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Viktor HaagSep 2 '10 at 18:27

Assumption #0: Vulnerability and resistance are 2 sides of the same coin and have a consistent application of concepts. Understanding one is at least instructive to adjudicating the other.

Assumption #1: For all resistances an vulnerabilities, the highest numerical value applies for any given damage type. Derived from "Not Cumulative", Rules Compendium (RC) pg. 224-226

Assumption #2: When determining damage, the damage is adjusted by the least resistance applicable and by the greatest vulnerability applicable. Derived from "Against Combined Damage Types", RC pg. 224-225 (Basically the damage always flows to where it will do the most damage.)

Assumption #3: Resistance or Vulnerability to "ALL" damage equates to a specific resistance/vulnerability to each damage type individually, as well as a specific resistance/vulnerability to untyped damage. Derived from "Combined with Resistance" pg. 226.

This last example under "Combined with Resistance" is particularly telling since the damage type is capable of activating each vulnerability by itself, but the damage is only modified by the greater of the two.

Skeolan has a well reasoned response to this question, but I have a objection to his second assumption. If the discrete damage was applied to each vulnerability in isolation, then the example on page 226 would result in 7 extra damage, not 5 damage as it describes in the RC. The application of multiple vulnerabilities stacking relies upon this assumption. In his defense, he answered before the Rules Companion was widely available, so he did not have the benefit of that change/example.

The interpretation of the rules that has the most consistency is that all vulnerabilities are considered in aggregate, not isolation and the damage is applied only once to the greatest applicable vulnerability.

This also mirrors the specific example of multiple damage types vs multiple resistance types under "Against Combined Damage Types" on page 224-225. Basically, resisting both types of damage dealt does not activate both resistances or provide additional protection the attack. Only the lesser resistance counts. In a similar way, being vulnerable to both types does not activate both vulnerabilities to make you more vulnerable, only the greater vulnerability counts.

Damage seeks to do the most damage, but each discrete instance of damage is only applied to one resistance and one vulnerability.

So to answer the original question (borrowing from Skeolan's breakout of case 2):

Vulnerability to a specific damage type applies even when that damage
type is combined with another. For instance, if a creature has vulnerable
5 fire, the creature takes 5 extra fire damage when it takes ongoing fire
and radiant damage.

Not Cumulative

Vulnerabilities to the same damage type are not cumulative.
Only the highest vulnerability applies. Example: If a creature has
vulnerable 5 psychic and then gains vulnerable 10 psychic, it has
vulnerable 10 psychic, not vulnerable 15 psychic. Similarly, if a creature
has vulnerable 5 psychic and then gains vulnerable 2 to all damage, the
creature still has vulnerable 5 psychic, not vulnerable 7 psychic

I'm sorry, but this in fact does not answer the question. Prior to writing my question, I had reviewed the article you mention as well as the current D&DI Compendium entry for "Vulnerable". The entry addresses how to handle a single vulnerability, when taking damage of single or multiple types. It does also mention how to stack multiple effects that apply vulnerability to the same damage type. However, it still does not address my question regarding how to handle multiple vulnerabilities when taking damage of multiple types.
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IsziSep 3 '10 at 16:37